Abstract: As long term usage of antibacterials,drug-resistant of germs increase,in order to slow down the generation speed and strengthen the effect of drugs,research on compatibility of medicines has been paid attention.In this study,broth dilution method was used to determine for the MIC （minimum inhibit concentration） of Lacti-trimethoprim and Mequindoc respctively,and their combinations,the effect of the combination was investigated by fractional inhibitory concentration （FIC） index.The result showed that the combination of Lacti-trimethoprim and Mequindoc took on the synergistic effect,the optimization rate was 1 ∶1 for V.anguillarum and V.parahaemolyticus,which was 1 ∶2 for V.alginolyticus.However,the present rate of Lacti-trimethoprim and antibacterials usually is （1∶3）~（1∶5）,which is not siutable for Mequindoc.The conclusion could be made that the combination of Lacti-trimethoprim and Mequindoc can improve antibacterial activity and decrease cost,it will useful to control the diseases which were caused by bacterial infection.